CD4 T cell responses are of critical importance for the control of viral infections in animals and humans. Yet surprisingly little is known about what constitutes a protective CD4 response as well as the mechanisms that lead to CD4 T cell failure. Recent technological advances in identifying and isolating virus-specific CD4 T cells and in analyzing the functional and transcriptional state of small numbers of cells now allow to answer basic questions about the role and fate of CD4 T cells in chronic versus acute infection, with the expectation that the results will enable rationale vaccine design as well as the development of targeted immunotherapeutic interventions. In order to better define critical aspects of the CD4 T cell response directly in humans, we will use the model of hepatitis C virus (HCV) infection. HCV infects almost 200 millions people worldwide and despite recent progress in antiviral therapies will remain a pressing clinical problem in the world for the foreseeable future. Thus alternative therapeutic modalities as well as prophylactic vaccines remain a high priority. Most importantly for this proposal, HCV allows to study both acute and chronic viral infection and thus protective and ineffective immunity, since an estimated 30% of infected subjects clear the virus spontaneously while the rest develop chronic infection and hepatitis. As a critical resource for our studies we have established a specimen bank with more than 30,000 PBMC and over 15,000 plasma samples from cohorts with more than 1000 HCV+ subjects, including almost 300 individuals within 6 months post infection. Based on our recent findings that virtually all subjects prime a measurable and typically multi-specific CD4 response upon HCV infection we hypothesize that functional differences between HCV-specific CD4 T cells define the outcome of HCV infection, that persistent infection gives rise to CD4 exhaustion through the activation of T cell inhibitory pathways and that the virus is able to circumvent the CD4 response by mutating towards less recognizable variants. Thus in aim 1 we will test whether CD4 responses in acute HCV infection differ in their capacity to secrete cytokines that help coordinate the different arms of the immune system and in the initiation of intracellular programs associated with T cell survival. Aim 2 will extend these studies by asking whether different functions and a different fate of the HCV-specific CD4 T cells is driven by the activatio of distinct T cell inhibitory pathways, with special consideration of intrahepatic CD4 T cells. In aim 3 we will define the role of viral escape mutations for persistence of HCV. At the end of the proposed studies we expect to have established a detailed understanding of how CD4 T cells can help to control viral infections and what differentiates protective CD4 immunity from a failed CD4 response. These results could have major implications for the prophylaxis and treatment of chronic viral infections, beyond HCV infection alone.